Heretofore, solvent extraction systems for removing organic contaminants from soil, sediment, sludges and debris have used distillation systems to purify the organic solvent process fluids. The organic solvents used in these systems are typically volatile, while the contaminants in the solvents are semi-volatile to non-volatile. During the distillation process, the volatile solvents are removed from the distillation column, and the semi-volatile and non-volatile contaminants are concentrated as still bottoms. The volatile solvent is condensed after leaving the distillation column and is reused in the system to extract more contaminants from soil. The still bottoms with the concentrated contaminants are removed and are typically shipped off-site for destruction at an approved facility.
The solvents used in these systems to dissolve the organic contaminants vary widely. They are selected for their ability to dissolve the contaminant of interest, usually have lower toxicity than the contaminant, and can be separated from the target contaminant by distillation. Because of the energy usage in distilling and condensing the solvent, compounds with low latent heat values are selected in order to minimize energy usage.
The contaminants collected by these systems include Polychlorinated Biphenyls (PCBs), chlorinated dioxins, chlorinated furans, chlorinated pesticides, chlorinated herbicides, wood preservatives, and other organic compounds. Nonvolatile compounds are most often processed by solvent extraction systems as these contaminants have no low cost remedial alternatives.
The distillation process has been successful in separation of contaminants from the solvent streams, and solvent extraction processes have been used to clean United States superfund sites and other contaminated sites throughout the world. Distillation systems, do, however, have several serious problems.
Distillation systems use large amounts of energy to volatilize the solvent, and also large amounts of energy to condense the solvent. Because of the heat involved, and the fact that many of the solvents used in solvent extraction systems are flammable, safeguarding the system to operate in an explosive proof manner is a large expense. The capital cost of distillation systems is also expensive. A full scale solvent extraction system may have a $1,000,000 distillation system. Due to the large capital expense, the size of the systems can not be altered to fit the individual site size, rather, sites have to be found that are in the size range of the distillation unit.
Distillation systems also require continuous monitoring, which drives the instrumentation and labor costs upwards.
Because of the high capital costs, energy costs, and labor costs associated with distillation units, solvent extraction companies have focused on large superfund type sites. Sites smaller than 10,000 tons have usually not been addressed by solvent extraction systems. Current solvent extraction methods using distillation have need for improved solvent purification systems that will lower capital costs, energy costs, and labor costs so that small and large contaminated sites can be processed with this technology.